1. Field of the Invention
This invention relates generally to the field of wireless communications and, more particularly, to antenna systems.
2. Description of the Related Art
Satellite communication systems are commonly employed to globally transmit data signals from an originating destination to a receiving destination. FIG. 1 shows a conventional satellite communication system. For an uplink operation, where communications signals are transmitted by a ground station 110 to a satellite 109, a data signal is first sent to a modulator circuit 112 in the ground station 110. From this data signal, modulator circuit 112 generates a modulated carrier signal with a frequency in one of the desired frequency bands. The modulated carrier signal is then sent to an input port on a waveguide assembly, commonly called an antenna feed 102. Antenna feed 102 is typically positioned such that its radiated output is efficiently coupled to a system of one or more reflector units 100. Antenna feed 102 acts as a transducer that converts the modulated carrier signal into radiated electromagnetic waves 114 that illuminate reflector unit 100. The electromagnetic waves 114 are then directed by the reflector unit 100 to satellite 109.
For a downlink operation, where communication signals are transmitted by satellite 109 to ground station 110, the above process occurs in reverse. Radiated electromagnetic waves of a modulated carrier wave are transmitted from satellite 109 to reflectors 100. The waves are redirected by reflectors 100 into antenna feed 102. Antenna feed 102 then acts as a transducer to route the received signals to appropriate receiver ports. Waveguide may couple the receiver ports to a demodulator circuit 112. Demodulator circuit 112 receives the carrier signal and recovers the data transmitted by satellite 109 by extracting the underlying data signal from the modulated carrier wave.
Satellite communication systems commonly employ more than one frequency band for electromagnetic signals radiated from a transmitting station to a receiving station through a satellite orbiting above the earth. These systems typically convey information on carrier signals in a number of different frequency bands approved by regulatory organizations and standards bodies (e.g., the Federal Communications Commission or FCC in the United States). Among the most widely implemented bands are the C band, X band and Ku band. These three bands together extend over two octaves of the communication frequency spectrum. The C band comprises frequencies in the range from 3.625 GHz to 6.425 GHz. The X band comprises frequencies in the range from 7.250 GHz to 8.40 GHz. The Ku band comprises frequencies in the range from 10.950 GHz to 14.500 GHz. The C, X and Ku bands are typically subdivided into many sub-bands wherein uplink and downlink data streams independently reside. Satellite communication systems employing single band communications are commonly referred to as narrow-band wireless signal communications. Multi-band communication systems are commonly referred to as broadband wireless signal communications.
FIG. 2 shows an antenna system 90 utilizing an antenna feed 102. Antenna system 90 includes a main reflector 100, a subreflector 101 and an antenna feed 102. Support member 105 supports subreflector 101 and antenna feed 102. Waveguides 107 connect antenna feed 102 to a plurality of transceivers 108. While three transceivers are shown, other combinations of receivers are possible. The use of subreflector 101 may be optional in some configurations. The main reflector 100, subreflector 101 and antenna feed 102 may be positioned in a prime focus, single offset, dual offset, Gregorian, Cassegrain, or Newtonian configuration. In the case of a prime focus configuration, subreflector 101 is removed. Main reflector 100 is typically paraboloidal, and subreflector 101 is typically hyperboloidal, but other shapes may also be used.
Prior art systems have typically relied on separate antenna feeds for transmission and/or reception of the C, X, and Ku frequency bands, i.e., a C-band antenna feed with its own input/output (I/O) port to transmit or receive in the C-band; a X-band antenna feed with its own I/O port to transmit or receive in the X-band; and a Ku-band antenna feed with its own I/O port to transmit or receive in the Ku-band. Since three separate antenna feed structures are needed, data transmission or reception in different frequency bands requires the physical removal of the first frequency antenna feed from the focal point of the reflector and the physical installation of a second frequency antenna feed into the focal point of the reflector. This movement is both a time consuming and tedious operation, in which improper alignment of the reflector and the antenna feed will cause distorted radiated patterns of the transmitted electromagnetic waves and may reduce transmission or reception efficiency. In addition, the distorted radiated patterns may be severe enough to violate FCC regulations. In order to prevent such problems, tests may be conducted after a switch is made from one antenna feed to another to obtain actual radiated patterns. This testing process may itself take several days to complete. Consequently, many ground stations limit their transmission or reception frequency to one of the three bands C, X and Ku. In addition, in the case of mobile satellite communications, there is a need for minimization of transportable payload weight in space or on earth. The use of multiple antenna feeds for communications at various frequencies may detrimentally increase payload weight and limit their usefulness on ground stations where size may be of highest importance.
Thus, a multi-band antenna feed structure capable of operating in two or more frequency bands simultaneously without the need for manual intervention is desirable. Such a feed structure may advantageously require fewer parts and consequently reduces depot supplies and training requirements. In the prior art, multi-band antenna feed structures have been recited. One such example is disclosed in co-pending U.S. patent application Ser. No. 09/183,355 filed on Oct. 30, 1998, entitled, xe2x80x9cA Method and Apparatus for Transmitting and Receiving Multiple Frequency Bands Simultaneouslyxe2x80x9d by Cavalier, et al., which is hereby incorporated herein by reference in its entirety. Cavalier, et al. teaches a multi-band antenna feed structure capable of simultaneous transmission and reception in the C, X, and Ku frequency bands. The structure, comprising coaxial waveguides and a subreflector, is preferably mated with parabolic reflectors. FIG. 3 shows a cross-section of one embodiment of a multi-band antenna feed 102 capable of transmitting and receiving C, X, and Ku frequency bands as according to Cavalier, et al.
When an antenna feed is designed for a reflector system, the matching of the antenna pattern to the angular aperture of the reflector is of primary concern. If the antenna pattern is too wide, the radiated electromagnetic energy spills over the edge of the reflector, and may result in reduced efficiency of the antenna system. This is commonly referred to as over-illumination of the reflector system. In addition, the energy lost due to the over-illumination result in side lobes that interfere with other neighboring antenna systems. Thus, stringent rules about an antenna""s spillover characteristics are enforced by the governmental agencies regulating the antenna systems. Conversely, if the antenna pattern is too narrow, the reflector is under-illuminated. This also results in reduced efficiency of the antenna system. The use of under-illuminated reflectors is generally avoided to minimize system cost and transportability. In addition, physical space constraints on the antenna system may prohibit the use of large reflectors. An ideally illuminated reflector matches the angular aperture of the reflector to the entire antenna radiation pattern being generated by the antenna feed, thereby providing optimum transmission and reception efficiency in the smallest footprint possible.
Traditional antenna feeds are typically designed for narrow band communications. They commonly employ collimating lenses or corrugated horns with the appropriate aperture size to produce the desired pattern beamwidth. Because they are designed to meet a specific beamwidth and frequency band, the antenna feed designs are relatively straightforward for one skilled in the art. Corrugated horns and/or collimating lenses have been used to assist in attaining the desired pattern beamwidth. However, the use of corrugated horns or collimating lens is not suitable for multi-band communications because their pattern beamwidth is a function of frequency. For example, if the pattern beamwidth being generated is ideal at one frequency, it is too narrow at higher frequencies and too wide at lower frequencies, resulting in poor illumination efficiency for multi-band communications.
The antenna feeds that have been designed for multi-band communications inherently generate broad pattern beamwidths, which severely limit their applications to prime focus reflector systems. For reflector systems requiring narrow beamwidth patterns, such as long-focal length single offset, folded double offset, and Cassegrain reflectors, these prior art broad-band, broad-beamwidth antenna feed systems are ill-suited to provide the desired optimum illumination efficiency. Accordingly, it would be highly desirable to provide a multi-band antenna feed system which produces narrow pattern beamwidths at multiple operating frequencies to maximize illumination efficiency and minimize the formation of side lobes. It would be further desirable to implement a multi-band, narrow beamwidth antenna feed system that avoids physical reconfiguration of the system for different operating frequencies and that minimizes the physical size of the system.
The problems outlined above may at least in part be solved by employing a non-collimating lens to produce narrow pattern beamwidths at multiple operating frequencies. Advantageously, an antenna system with such a lens may be able to transmit and receive broadband wireless signals with closer to maximize illumination efficiency of many reflector configurations. Such an antenna system may also minimize the formation of side lobes. In addition, the system may avoid the need for physical reconfiguration of the system for different operating frequencies, and it may reduce the system footprint by eliminating the need for a plurality of antenna feeds to handle the different operating frequencies.
A method for simultaneously transmitting and receiving broadband wireless signals is contemplated. In one embodiment, the method comprises generating a broadband wireless signal with an antenna feed and propagating the signal through a non-collimating lens. In one embodiment, the antenna feed is a tri-feed antenna feed. The lens is configured to focus the broadband wireless signal in a non-collimating manner and reflect the focused signal with a reflector for transmission. The method further comprises reflecting a received broadband wireless signal from the reflector and propagating the received signal through the lens. The lens is configured to focus the broadband wireless signal in a non-collimating manner to the antenna feed system. In one embodiment, the lens may be a planar convex configuration. In another embodiment, the lens may be meniscus. In some embodiments, the lens is configured to be attached to the front end of the antenna feed system. In other embodiments, the lens is configured to be attached in a cavity of the front end of the antenna feed system. The front end of the antenna feed system is the location where broadband wireless signals are both transmitted and received. In one embodiment, the lens may be formed of Rexolite. In other embodiments, the lens may be formed of fused quartz, teflon, polyethylene, or other materials.
A system for simultaneously transmitting or receiving broadband wireless signals is also contemplated. In one embodiment, the system comprises an antenna feed, a lens and a reflector. For transmitting, the antenna feed is configured to propagate the signals through a non-collimating lens. The lens is positioned to receive and focus the signals from the antenna feed to a reflector, which in turn may be positioned to receive and reflect the focused signal from the antenna feed. For receiving, the reflector is positioned to receive and reflect the signal through the non-collimating lens. The lens is positioned to receive and focus the signal from the reflector to the antenna feed, which is configured to propagate the focus signal from the lens.
A system for increasing sensitivity for a wireless sensor is also contemplated. In one embodiment, the system comprises a non-collimating lens configured to receive wireless signals and focus the signals onto a sensor. The sensor is positioned to receive the focused signal once it has passed through the lens. In one embodiment, the lens may be part of a nose cone, and the wireless sensor may be part of a navigational control unit for a missile. In one embodiment, the lens may have a planar convex configuration or a meniscus configuration. Advantageously, using the lens the missile may be able to detect electromagnetic radiation sources at farther distances and may be able to detect lower level electromagnetic radiation sources. In one embodiment, the lens may be formed of Rexolite. In other embodiments, the lens may be formed of fused quartz, teflon, polyethylene, or other materials.